Disk diffusion and MIC lab
Open your materials, follow the steps, then turn in your work.
Run a disk-diffusion test and connect zone of inhibition to minimum inhibitory concentration.
1. Open your materials
Use the materials named in the first step below. Open lesson resources.
2. Start the work
Put on goggles and gloves and confirm your plate is labeled with each antibiotic disk.
Show all 6 required steps
- Put on goggles and gloves and confirm your plate is labeled with each antibiotic disk.
- Place antibiotic disks on the bacterial lawn using aseptic technique.
- Set the plate to incubate and write what a clear zone around a disk will mean.
- Once zones form, measure each zone of inhibition in millimeters.
- Compare each zone to the breakpoint table for that specific antibiotic, and record susceptible, intermediate or resistant.
- Define MIC, and explain why a larger zone hints at a lower MIC for the SAME drug but says nothing when you compare two different drugs.
Lost your place? If you stepped away, check your plate: are the disks placed and is it incubating? If zones have formed, pick up at measuring each zone of inhibition in millimeters, then rank the drugs by zone size.
Check your work before submitting
- You will be able to set up a disk-diffusion test safely.
- You will be able to measure a zone of inhibition.
- You will be able to read a zone against its breakpoint table, and say why zone sizes from different drugs cannot be ranked against each other.
Before lab work: read the safety rules
- Goggles and gloves on before opening any bacterial culture material; keep on until cleanup is complete.
- Treat all bacterial materials as biohazardous: no contact with skin, eyes, or mouth.
- Open plate lids only briefly and only near the workspace; do not talk, sneeze, or cough over an open plate.
- Incubate plates inverted (lid down) to prevent condensation dripping onto the lawn.
- Before disposal: flood plates with 10% bleach solution and seal in a biohazard bag; autoclave if available.
- Wash hands thoroughly with soap and water after removing gloves.
3. Turn in your work
DueCheck Schoology- Hand in
- Disk-diffusion data table: antibiotic name, zone of inhibition measurement (mm), and effectiveness ranking; plus one sentence connecting zone size to MIC.
How to submit and name your file
Bring data table to Thursday's resistance analysis; photograph plate for portfolio.
In Schoology, open your course and the assignment for this lesson. Attach your file, select Submit, and check that it appears in the submission.
PDF upload helpYou get two school days for every day you were absent, so this deadline moves with you.
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How this lesson connects
Keep using what you learned last class: Because every use of an antibiotic kills the susceptible bacteria and leaves the resistant ones to multiply, appropriate use is a shared responsibility so that the drug still works when someone truly needs it. The drug itself does not weaken; the bacterial population changes. Today: A wider clear zone means this organism is inhibited at a lower concentration of that same drug, but zone width cannot rank one drug against another, because each antibiotic diffuses through agar at its own rate and each has its own breakpoint.
Optional: listen or watch a unit review▸
Need help? Warm-up, timing, and directions▸
💡 Big idea: A wider clear zone means this organism is inhibited at a lower concentration of that same drug, but zone width cannot rank one drug against another, because each diffuses through agar at its own rate and each has its own breakpoint.
- 0-8 minDon goggles and gloves; confirm plate is labeled with the abbreviation for each disk location
- 8-22 minUsing , place disks on the bacterial lawn at the labeled positions
- 22-30 minSet plate to incubate (inverted); write in notebook what a clear zone will indicate
- 30-50 minAfter (or using pre-incubated plates): measure each in millimeters with a ruler
- 50-65 minRank antibiotics by zone size from largest to smallest; record in a
- 65-80 minDefine MIC in notebook; write one sentence connecting larger zone to lower MIC and greater effectiveness
- • The test has been the clinical standard for sensitivity testing since 1966 and is still used in hospitals today.
- • The data you collect today will drive your resistance and stewardship analysis on Thursday and your report on Friday.
- • is critical: will produce zones that mean nothing.
- • Exit goal: a of zone measurements in millimeters for each disk, with an effectiveness ranking.
- • The test measures effectiveness by placing antibiotic-impregnated disks on a bacterial lawn and measuring the clear zone that forms.
- • The is the area around the disk where bacteria could not grow; a larger zone indicates the diffused farther and was effective at lower concentrations.
- • The minimum inhibitory concentration (MIC) is the lowest concentration of an that prevents visible bacterial growth; a larger zone correlates with a lower MIC.
PLTW connection and today's work
Open Activity 1.2.1 Antibiotic Therapy in myPLTW and apply your mechanism knowledge to the case patient.
Today's stopping point: Mechanism chart should be done (Tuesday); antibiotic selection and justification due today.
PLTW activity titles identify the course connection. If your account will not open, use the posted materials for today and tell Mr. Mendoza. Do not mark an online activity complete unless you completed it.
Use the turn-in directions at the top of this page. Do not create a second submission unless your teacher asks for one.
Show another explanation or a smaller first step
Need help? Choose a starting point
Finish the assigned lab safely before starting extra practice.
Lesson resources: reading, slides, and vocabulary▸
The deck carries the prior idea forward, lets you inspect an analogy, maps the rule to biology, and ends with the same evidence decision and exit ticket used on this page.
Generated from this lesson's canonical data with a red-team citation check.
Because every use of an kills the susceptible bacteria and leaves the resistant ones to multiply, appropriate use is a shared responsibility so that the drug still works when someone truly needs it. The drug itself does not weaken; the bacterial population changes.
A wider clear zone means this organism is inhibited at a lower concentration of that same drug, but zone width cannot rank one drug against another, because each diffuses through agar at its own rate and each has its own breakpoint.
A pharmacist converts a medication concentration in a bottle into a patient-specific dose before comparing it to the safe limit.
- What is the concentration?
- What is the intake volume?
- What is the final dose?
A threshold applies to the dose actually received, not just the concentration in the environment.
A single dose calculation assumes a one-time exposure and does not account for over time.
- • The bottle concentration maps to the environmental measurement.
- • The patient dose maps to the exposure actually taken in.
- • The pharmacy check maps to comparing against the regulatory threshold.
Driving question: You place four disks on one bacterial lawn and get clear rings of different sizes. How do you turn those millimeter measurements into a ranking of which drug works best?
What you already know: Because every use of an kills the susceptible bacteria and leaves the resistant ones to multiply, appropriate use is a shared responsibility so that the drug still works when someone truly needs it. The drug itself does not weaken; the bacterial population changes.
New idea: A wider clear zone means this organism is inhibited at a lower concentration of that same drug, but zone width cannot rank one drug against another, because each diffuses through agar at its own rate and each has its own breakpoint.
Visual or model: F1. F1. A lesson illustration or teaching diagram for Disk diffusion and MIC lab. Use it with E1-E3; it is a model or context image, not experimental or patient data. What to notice: Trace the labeled testing, treatment, or biological process and identify where evidence limits the decision.
- Observe or measure the relevant feature in today's lesson.
- Organize the observation with a stable evidence ID.
- Apply this rule: A threshold applies to the dose actually received, not just the concentration in the environment.
- Choose the option the evidence supports and state the limit of the conclusion.
Real biomedical example: You place four disks on one bacterial lawn and get clear rings of different sizes. How do you turn those millimeter measurements into a ranking of which drug works best?
What the evidence supports: E1-E3 and F1 support the daily take-home when the response meets the stated success criteria.
What it cannot prove: The package does not support claims beyond this lesson's or any real patient diagnosis.
Use it now: Choose one decision option. Cite E1 and E3, then explain how the rule connects the evidence to your choice.
Go further, optional: The source links below are optional enrichment. Every fact required for today's local evidence decision appears in this lesson package.
Antimicrobial resistance can arise and spread through selection and genetic change, so susceptibility results must be interpreted with method-specific standards and clinical context.
Limit: A zone size or classroom result alone does not determine a patient's treatment or prove a specific resistance mechanism.
A threshold applies to the dose actually received, not just the concentration in the environment.
Limit: A single dose calculation assumes a one-time exposure and does not account for over time.
You can set up a disk-diffusion test safely.
Limit: E3 defines the classroom product or success criterion. It is not independent scientific evidence and cannot justify a clinical or causal claim.
PLTW-GEND-2026-09-30 · Simulated classroom evidence scenario
Your role: medical interventions team member
Decision: Your team must decide what the evidence from today's lesson supports before submitting the labeled and result claim named on today's page.
- • Wait for the clinical context, since a plate cannot show how this drug behaves inside an actual patient.
- • Recommend a drug only after chaining mechanism, your zone data, and how resistance could erase that advantage later.
- • Name the with the largest zone and finish the report, because the measured data speaks for itself.
Response: State one choice, cite at least two evidence IDs, explain the rule that connects them, and add one limitation. Submit it as the labeled and result claim.
Claim ceiling: Today's evidence supports a classroom claim about today's lesson. It cannot prove causation, diagnose a real patient, or justify action outside this room.
Reason for review: Your team must decide what the evidence from today's lesson supports before submitting the labeled and result claim named on today's page.
Context: The clear zone around an disk is a measurable readout of drug strength, because a wider zone means the drug stops bacteria at a lower concentration.
- • T1: Put on goggles and gloves and confirm your plate is labeled with each disk.
- • T2: Place disks on the bacterial lawn using .
- • T3: Set the plate to incubate and write what a clear zone around a disk will mean.
- • T4: Once zones form, measure each in millimeters.
- • T5: Compare each zone to the breakpoint table for that specific , and record susceptible, intermediate or resistant.
- • T6: Define MIC, and explain why a larger zone hints at a lower MIC for the SAME drug but says nothing when you compare two different drugs.
- • E1: Antimicrobial resistance can arise and spread through selection and genetic change, so susceptibility results must be interpreted with method-specific standards and clinical context.
- • E2: A threshold applies to the dose actually received, not just the concentration in the environment.
- • E3: You can set up a disk-diffusion test safely.
Measurements: Use only the measurements, units, graph, or counts supplied in today's task. No additional patient measurement is implied.
Figure finding: Teaching diagram for and MIC lab. Trace the labeled testing, treatment, or biological process and identify where evidence limits the decision. This is a teaching model, not patient or experimental data.
Uncertainty: This is a composite classroom scenario. Missing history, measurements, or confirmation tests remain unknown and limit the conclusion.
= final volume / sample volume. New concentration = starting concentration / dilution factor.
Mix 1 mL of sample to a final volume of 10 mL. The is 10. A 100 mg/mL starting sample becomes 10 mg/mL.
Use the same volume units before dividing. Concentration keeps its original concentration unit.
Apply the same setup to one supplied dilution or dose. Show the factor, new value, units, and a reasonableness check.
- • The solution must address the stated need in today's lesson.
- • The decision must be supported by E1-E3.
- • The final product must make the success criteria visible.
- • Complete the work inside the 80-minute block.
- • Use only supplied or teacher-approved materials and evidence.
- • Do not trade , accessibility, or privacy for speed.
- • and evidence quality: must pass before scoring other criteria.
- • User need and effectiveness: highest scored criterion.
- • Time, cost, and ease of use: compare only after and effectiveness pass.
Test evidence: For each option, record the E1-E3 result that supports or fails each criterion. Do not assign a score without a named observation.
- Version or option tested
- Criterion met or missed
- Evidence ID and result
- Revision made
- Reason for the revision
- Need and user
- Criteria and constraints
- Chosen option and evidence
- Test result
- Revision and reason
Students often think Many students think a bigger clear zone means the drug needs a higher dose, since it seems to be doing more work.. The trap: That is a trap because it reverses the relationship. A bigger zone means the drug stopped growth even where it had diffused thin, so it works at a lower concentration, which means a lower minimum inhibitory concentration, not a higher one.
Connection to MIC: a larger zone of inhibition means the antibiotic stopped bacterial growth even where it had spread thin and dilute, which points to a lower minimum inhibitory concentration (less drug needed to work).
Comparison, with its limit: C produced the widest zone (22 mm), then A (18 mm), then B (9 mm). Zone width cannot rank different antibiotics against each other, because each drug diffuses through agar at its own rate and each has its own clinical breakpoint, so this ranks the zones, not the drugs.
| Antibiotic | Zone of inhibition (mm) | Effectiveness rank |
|---|---|---|
| Antibiotic C | 22 | 1 (most effective) |
| Antibiotic A | 18 | 2 |
| Antibiotic B | 9 | 3 (least effective) |
This model shows the level of evidence and organization needed to complete: A data table listing each antibiotic, its measured zone of inhibition in millimeters, and an effectiveness ranking, with a sentence connecting zone size to minimum inhibitory concentration.
- Name the variables and include units.
- Enter observations without changing the raw values.
- Check labels, calculations, and patterns before interpreting the data.
Keep the structure. Replace the question, facts, measurements, and evidence. Then recheck units, vocabulary, and whether the conclusion goes beyond the evidence.
Also due today: Bring data table to Thursday's resistance analysis; photograph plate for portfolio.
- CER:
- Claim, Evidence, Reasoning: make a claim, back it with evidence, explain your reasoning.
- SOP:
- Standard Operating Procedure, the exact steps to follow (especially in a lab).
- Tracker:
- Your PLTW progress log where you record completed evidence.
- myPLTW:
- The PLTW course site where you do the online activities. Find it in Clever with your Microsoft sign-in, right next to Schoology.
Classroom documents for this lesson are posted in Schoology. Open Clever, then Schoology, and find each one by the name shown on its card.
Open this when the class reaches this activity and use it to complete the required lesson artifact.
Placement rationale
Matched treatment, MIC, resistance by path:Medical-Interventions/Unit-1_How-to-Fight-Infection/1.2_Antibiotic-Treatment; keywords:antibiotic, therapy. Score 142. Visibility: student-schoology (student-facing resource; link through Schoology rather than local path).
Open this when the class reaches this activity and use it to complete the required lesson artifact.
Placement rationale
Matched treatment, MIC, resistance by path:Medical-Interventions/Unit-1_How-to-Fight-Infection/1.2_Antibiotic-Treatment; keywords:antibiotic, resistance. Score 142. Visibility: student-schoology (student-facing resource; link through Schoology rather than local path).
Use this if you were absent, got stuck, or need another pass before you submit the lesson artifact.
Placement rationale
Matched treatment, MIC, resistance by path:Medical-Interventions/Unit-1_How-to-Fight-Infection/1.2_Antibiotic-Treatment; keywords:antibiotic, resistance. Score 138. Visibility: student-schoology (student-facing resource; link through Schoology rather than local path).
Sign in to Clever with your district Microsoft account to open Schoology or myPLTW. Follow today's posted steps. If myPLTW will not open, use the posted alternative and tell Mr. Mendoza. Turn in your completed work through the Schoology assignment.
Practice: try a question, then check your answer▸
Claim ceiling for this check: Today's evidence supports a classroom claim about today's lesson. It cannot prove causation, diagnose a real patient, or justify action outside this room.
Drug A leaves a 22 mm clear zone and Drug B leaves a 10 mm zone on the same bacterial lawn. Which drug has the lower MIC, and what does that mean?
Write an answer and pick a confidence to unlock the key.
Missed class or ready for more?▸
Run this before you touch the bench. It is built from the real lab procedure, so the decisions you make here are the ones you will make with the equipment in your hands.
I can name the procedure's purpose and the evidence I will record. I can name today's hazards and the control for each: Goggles and gloves on before opening any bacterial culture material; keep on until cleanup is complete. My data table is ready before materials are handled.
Finish the checklist before you handle any material.
- • Goggles and gloves on before opening any bacterial culture material; keep on until cleanup is complete.
- • Treat all bacterial materials as biohazardous: no contact with skin, eyes, or mouth.
- • Open plate lids only briefly and only near the workspace; do not talk, sneeze, or cough over an open plate.
- • Incubate plates inverted (lid down) to prevent condensation dripping onto the lawn.
- • Before disposal: flood plates with 10% bleach solution and seal in a biohazard bag; autoclave if available.
- • Wash hands thoroughly with soap and water after removing gloves.
- 1Before materials are handled, identify the purpose, variables or comparison, controls, measurement units, and stop-work condition.
- 2Put on goggles and gloves and confirm your plate is labeled with each antibiotic disk.
- 3Place antibiotic disks on the bacterial lawn using aseptic technique.
- 4Set the plate to incubate and write what a clear zone around a disk will mean.
- 5Once zones form, measure each zone of inhibition in millimeters.
- 6Compare each zone to the breakpoint table for that specific antibiotic, and record susceptible, intermediate or resistant.
- 7Define MIC, and explain why a larger zone hints at a lower MIC for the SAME drug but says nothing when you compare two different drugs.
- 8Record each result in the prepared table before interpreting it. Mark missing, repeated, or invalid results truthfully.
- 9Complete the named cleanup and waste route, remove PPE safely, wash hands when required, and confirm the station is ready for the next group.
| Trial or sample ID | Independent condition | Measured result with units | Observation before interpretation | Quality-control note |
|---|---|---|---|---|
Before the procedure, predict the result and cite the rule behind the prediction.
After the procedure, compare the result with the prediction and name one limitation or source of uncertainty.
What today's skills lead to. These are real health-science careers this course builds toward. Tap one to see, on the US Department of Labor's O*NET site, what the job actually involves, what it pays, and how fast it is growing.
If you miss the lab, analyze the teacher disk-diffusion plate images, measure each in millimeters, and submit your ranking of effectiveness.
CDC Antimicrobial Resistance resourcesBring data table to Thursday's resistance analysis; photograph plate for portfolio.
Class still runs. Complete the online activity above (it's self-guided). Need the concept taught without a teacher? Use this authoritative explainer:
CDC Antibiotic Resistance- CompleteEvery required part of the artifact is present, nothing left blank.
- AccurateThe science and the data are correct and match the evidence.
- Scientific reasoningYou explain your claim with evidence and reasoning (CER), not just an answer.
- Professional communicationClear, organized, labeled, and written the way a clinician or scientist would.
- SubmittedGo to Schoology to turn this in. Submit one PDF. Put your first and last name in the document header. Name the file: FirstName LastName - Assignment Title - YYYY-MM-DD.pdf. If you cannot get in, see Mr. Mendoza. Do not skip the work.
- Error analysis and method · counts doubleName a specific limit of the method and how it moved your result, and compare what you predicted to what happened. "Human error" does not count; say what about the procedure or instrument caused it.
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